Heretofore, microcapsules have been prepared by a method that utilizes the phenomenon of coacervation of gelatin and gum arabic; a method that depends on the interfacial polycondensation of a polyisocyanate and a polyamine, a polyisocyanate and a polyol, or a polybasic acid chloride and a polyamine, or a method which involves the in situ polymerization of a polyisocyanate and a polyol, polymerization of styrene monomers, or the in situ polymerization of melamine- or ureaformaldehyde. These prior art microcapsules are simply intended to protect the confined core material or expose it by rupturing the capsules under pressure. It suffices that the capsule wall satisfies the following physical requirements: the wall normally prevents the liquid core material from going out of the capsule but can be ruptured under pressure to permit the exposure of the core material as required; or the thickness and the porosity of the wall are so controlled that it permits gradual release of the core material. In order to meet these requirements, various materials have been reviewed with regard to their use as microcapsule walls and many types of microcapsules have been prepared by interfacial polycondensation or in situ polymerization which is intended to make capsule walls of polyesters, polyamides, polyurethane, polyurea, urea/formaldehyde resin or melamine/formaldehyde resin.
Attempts have also been made to prepare light- or heat-sensitive microcapsules which contain materials that are decomposed by light or heat to generate gases which break the capsule wall to permit the exposure of the core material.